Petroski on Engineering: Armchair Design & Analysis

The tray table that folds in half for stowage in the armrest of an airline seat is something I have long admired for its design ingenuity, but long cursed for its operational opaqueness and flimsiness. On domestic flights, tables of the kind I am describing are usually found in the first-class cabin and on seats facing a bulkhead. The more common rigid tray tables that swing down from the back of the seat in front of a passenger are obviously less costly to manufacture and install, and so are used wherever the seat pitch allows them.

But even before the more complex armrest table can be deployed into a (hopefully) flat and horizontal position, the airline passenger has to get the thing out of its underarm pocket. This can be more easily said than done, for there is no standard feature to grasp on the stowed table. Some models have a fabric loop, some a trigger-like hook, and some a corner hole. Since not all passengers are able to contort their bodies or squint their eyes sufficiently to see what aid might be lurking in the dark recess, they tend to feel around for a familiar shape and tug on it.

Once the table has been wrestled out of its hold, it can typically be rotated about an orthogonal axis to become a half table, often with a slight depression in its surface to keep drinks from walking off the edge during air turbulence or just under ambient vibration. It is in deploying this half table that passengers might first discover that the top can have a pronounced incline. On a recent flight from Chicago to Raleigh-Durham I found myself with such a table. As I waited for the flight attendant to bring me a drink, I almost put the table back into the armrest, since I would have had to hold the glass anyway.

The detailed design of the kinematically clever deployable armrest table understandably varies from airplane model to airplane model, but virtually all variations seem to depend for their operation on two primary mechanical principles: the hinge and the cantilever. Some also contain a slide or swivel feature, which enables the more corpulent user to push the tray forward or aside to gain some breathing room.

In one common form, the two halves of the table have their top edges connected through a pair of link hinges; the bottom edges of the two halves butt up against each other, providing reaction forces. Collectively, the forces maintain a cantilevered table top in a flat and horizontal position. One familiar form of tray table, when fully deployed, forms a bridge of sorts between a seat's armrests.

On my flight where the folded table sloped downward, the unfolded one would also have -- had it not been for the opposing armrest providing a support constraint. Thus, the fully opened table top formed not a flat plain but a shallow valley. The two halves inclined toward the center, creating a V into which everything from pencils to peanuts to plastic drink glasses would want to slide.

Entertaining musing on the give and take of good design. What's the greater takeaway for engineers, though? Don't factor use case into your efforts for fear of legal repercussions? That seems like a cop-out to me. After all, not every flight enjoys the benefit of having mechanically-inclined flight attendents on board.

It's been years, but I remember a flight with, I swear, that same tray table "design," which made it completely unusable when deployed. And that was so obviously the case that I could not understand how the design ever got accepted and the tray table installed, in thousands of planes. I was not so fortunate as Professor Petroski, however, in my flight attendants. This all makes me think less of the legal issues than of the designers/users and use case issues.

Ann, Beth, Dave & Prof. P., I write this as you Professor wrote your article on arm chair design....with toung in cheek. The story starts with me being the only person, other than crew, left on a plane durind a short layover. A AL Mechanic arrived and went to work on one of the tables you all described. He almost instantly recived a loud message over his 2way "you have 5 minutes". He started to work very delicatly and expertly for the next 10 minutes when he received a VERY loud call "you need to pack it in we have to load."

Being a company man (I determined by his appearence) he reached down to his tool belt, holstered his Phillips Head screw driver swept his hand dirrectly to the hammer loop, grabbed it , pounded the table into the midseat container, flipped the cover closed and annouced towards the Captain "All fixed sir she's ready to go have a good flight" while exiting the aircraft.

THIS ALL HAPPENED FASTER THAN THE TIME IT TOOK ME TO WRITE THIS. It sticks in my mind to this day because I am also a Pilot.

The story of the mechanic pounding the broken table into submission is great! It shows how the human element can be the ultimate enemy of designs of all kinds. I wonder how long it took that tray table to finally be fixed so it could be both gotten out of and put back into the seat arm? Poor maintenance can be worse than no maintenance at all.

The maintenance story reminds me of an episode of the cartoon Animaniacs in which two of the characters are trapped in an elevator. They try to contact the maintenance department over the intercom, and overhear the supervisor telling one of his employees: "Hit it with the hammer, Big Ed. No, the hammer... that's a wrench, that long thingy's a hammer..."

Later, after being trapped in the elevator for ten hours, the characters try to contact the maintenance department again.

"Are you still in there?" the maintenance supervisor replies. "It was our indication that you got out."

One thing I don't think the airlines or designers really considered is the lifetime of the product in this case. Is it reasonable to expect something that gets as much use as a tray table to last the lifetime of the plane. Those are typically flown for many years. They should have designed them to easily replaced (say during a layover) and refurbished in the shop.

Why not just make the tray from a sturdy piece of corrugated cardbord coated with plastic. Advertise a ski resort on one side and a Maui hotel on the other. Clipped securely to BOTH armrests it would hold more weight than a cantilevered tray, weigh about an ounce, cost 97 cents, and stow in a pocket on the seatback in front of you. A tray with no hinge is harder to break, but if somebody breaks it anyway, the airline gets to sell more advertising.

I think the airlines' issue with your suggestion, Russell, is that if I'm reading it right, your design would be loose. Yes, it's connected to both armrests when the passenger wants it to be, but if the passenger is putting it back, or doesnt' secure it properly, or whatever, it becomes the airlines responsibility if it goes flying in turbulance.

The cardboard tray wouldn't weight much more than the plastic plate sitting on it, and unlike the plate, the tray would normally be secured. Even if left unsecured the tray wouldn't be much of a projectile, but adding foam-rubber edging would limit the damage when kids play with the trays. In normal use the edging would also keep cups and plates from sliding off.

Professor Petroski's final point in this article alludes to the fact that product liability law requires a product to be safe not only in its reasonably expected use, but also any reasonably expected misuse. Engineers are often horrified to learn that the law requires us to take potential misuse of a product into account. Ironically, although we bristle at the idea of other people misusing our products, we also excel at coming up with creative new ways to "use" products others have designed. Some of the new uses which particularly creative engineers find for things may not fall into the "reasonably expected" category.

I've had another rude awakening this week: no matter how hard I try, how intelligent I may be; there is always one idiot "smarter" than me who will discover the way to damage the machine. This week, the idiots won.

Arlo Guthrie has a thing or two to teach engineers about "reasonably expected misuse", from his song Alice's Restaurant:

"Kid, I'm going to put you in the cell, I want your
wallet and your belt." And I said, "Obie, I can understand you wanting my
wallet so I don't have any money to spend in the cell, but what do you
want my belt for?" And he said, "Kid, we don't want any hangings." I
said, "Obie, did you think I was going to hang myself for littering?"
Obie said he was making sure, and friends Obie was, cause he took out the
toilet seat so I couldn't hit myself over the head and drown, and he took
out the toilet paper so I couldn't bend the bars roll out the - roll the
toilet paper out the window, slide down the roll and have an escape.

That's a great song, TJ. He really nailed a lot of truths in the song. I saw him a couple of years ago at a festival. He still does "Alice's Restaurant." I guess he's stuck with it. I saw him sing it a couple times in the 1970s. That was understandable. But I was surprised he was still singing it 40 years later.

The nice thing about the tray tables that Professor Petroski describes is that they are not attached to the seat back. Anything is better than that. If I put my laptop on one of those, the person in front of me glances back over his/her shoulder to let me know I'm bothering them. Worse, when the person in front leans back, the tray table is suddenly so close that there's no room to work.

The lowly fold-out tray table is a pretty amazing piece of design. Besides the basic function, there's numerous secondary requirements. It must be lightweight, strong, and meet flammability requirements. It has to support a certain amount of weight but break away if someone hits it hard without leaving sharp pieces and remain able to be stowed after being broken. It can't have any pinch or "guillotine" hazards and can't fly out of the bin in a 15g foward-aft impact.

Airplane tables are useful... as mouse pads. I had access to a bunch of old trays being thrown out. For black or featureless tables they make very nice mouse pads! No bending or warping. Conversation piece too.

As for use on aircraft, they do leave a lot to be desired. However, there is little choice. Make a great table and someone will lean on it and bend it. Make it stronger? Not only heavier but more tempting to lean on. Live with it - and carry a pop can tab.

I have come across those tables that pop up from between the seats and have never had any problems with them. Probably being an experienced engineer helps a bit there. I have been on lots of flights where I had to hold my drink so that the bumps would not launch it into my lap, or somebody else's lap. I agree that the ones that fold out of the back of a seat are much nicer, although I have never been able to use a computer on a plane.

If you want a real challenge though, try to utilize the footrest on one of the Amtrack trains. There is a button on the armrest to release it, but in the trains that I have ridden there is nothing to make it move once it is released. The only way we figured out to use the footrest is to grab it with a hand and pull it into position. My guess is that the springs were removed to prevent terrorists from somehow utilizing the footrests for something, or possibly to prevent idiots from doing something really stupid with them.

I really would preferr that our legal system stop rewarding idiots and fools for injuring themselves, and instead, fine them for being so stupid. OF course, then we would have the lawyers seeking customers under some other premise, and that might be worse.

I had to check the date of this commentary - 2012, not 1965! In the '70s I worked in aircraft seating design. Those First Class seats were a wonder of engineering. Their weight-saving, weight-supporting design were worthy of awards. I'd think the present-day comments would have been unwarranted, even back then. Surely engineering has progressed since...or has it?

Airlines seem to have put their seating R&D dollars into improving the business and first-class sections on international flights, where seats transform into beds of a sort. Econmony class seat design does seem to have been static for some time. There has been, however, some improvement in leg room for a price to the customer. With the increasing prominence of narrow-body regional jets, it seems unlikely that economy-class seats will get wider. There seem to be simply too many constraints from available space and revenue to allow for much improvement.

Thanks Henry. Yes, I understand the economics of space for the back of the plane. And the space has grown smaller over my decades flying. The basic design of tray tables, arm rests, seatbelts, etc. has also been static for decades. I would guess the airlines are simply satisfied with these design elements.

The airlines may be satisfied with the design of the interior elements of their planes, but of course that does not mean that there is not room for improvement. The main selling point for economy air travel seems to be ticket price. Passengers seem to give that much greater priority than amenities relating to seat comfort. My feeling is that until passengers in large numbers let it be known that they are not satisfied with what the flying experience is like, the airlines will not redesign it beyond changes that bring in more revenue, such as more leg room, etc.

Yes, Henry, I agree that ticket price is paramount for air travel consumers -- including myself. And I can't imagine travelers choosing on airline over another based on anything but price. So I guess that says it all. No reason to improve comfort.

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